Coil voltage drop

Why are you trying to measure that voltage? Is there a running problem you're trying to solve?

You have a number of things working against you. The first is that resistor or resistor wire that's used with breaker points. When the points are turned on and current is flowing, that current is going to cause a voltage drop across the resistor, leaving you with less than full system voltage for that instant. Then, when the points open, current flow stops, that voltage drop across the resistor goes to 0 volts, leaving you with the full charging system voltage, which should be between 13.75 and 14.75 volts.

All generators are inefficient at lower speeds, so we expect charging voltage to drop a little at idle, and when a number of other loads are turned on, especially high-current loads like head lights and heater fans. That means system voltage applied to the ignition system is constantly varying too.

To add to the misery, when the points close, current flow can't go to full flow instantly because it takes time for the magnetic field to build up in the ignition coil. It takes energy to do that, and that energy opposes the current flowing through the coil. Current flow actually ramps up relatively slowly. That's called the "dwell time". That same slowly-rising current causes a slowly-rising voltage drop across the resistor. To say that a different way, the voltage at the coil's positive post will drop gradually each time the points close.

All of that has no special meaning to us except when you put a digital voltmeter in there. They take a reading, analyze it, then display it while it takes another reading. Typically that happens two or three times per second. One time it might see 10.2 volts, the next time it might be 6.5 volts, and then, 8.8 volts. Those readings will jump around with no rhyme or reason. If you really want to know the average voltage, you need the older pointer-type voltmeter.

When it stalls, don't try to restart it right away. Look down the carburetor to see if there's a nice strong squirt of fuel when you work the throttle. If you don't see that, a real good suspect is a plugged strainer on the fuel pick-up tube in the gas tank. The clues are the stalling occurs gradually, over a few seconds, along with some surging, and the engine will usually restart within five to ten minutes, then stall again within a few miles.

Electrical-related causes usually occur instantly, like if you turned off a switch, and when it's heat-related, they take up to an hour to cool down, then the engine will run again.

Unlike on newer fuel injected vehicles, at least on GM and Chrysler products, that strainer just slides onto the end of the pick-up tube. I had to replace two of these on two of my carbureted Chrysler products, but for the one GM I was involved with, my instructor did indeed blow it off with compressed air at the hose connected to the mechanical fuel pump. Be aware though, you want to remove the fuel cap so the air can escape without blowing up the tank.

Once this solves the problem, consider cutting the supply line and inserting an inline Chrysler fuel filter. Most GM products used a sintered metal filter that looked like a porous rock, sitting inside the housing on the carburetor where the fuel line screws in. Those plug up very easily and often, and now with no strainer in the tank, that is going to be a monthly service. Chrysler fuel filters typically last the life of the car unless they rust out and start leaking.

If the engine suddenly starts running normally again, that doesn't sound like the strainer is the cause of the stalling. When they plug up, you'll just go slower and slower until walking becomes the faster alternative.

That is obviously not good, but it shouldn't be related to a loss of fuel or spark. That wire is the ground for the exterior lights, radio, horn, and things like that. The ignition system is grounded to the engine, then that returns through the fat negative battery cable. If that fatter cable is badly corroded under the insulation where you can't see it, or if it has been moved off the engine, you would have a cranking problem. The engine, transmission, and exhaust system all sit or hang on rubber isolators. That's why that fat negative cable needs to be bolted to the engine.

If the smaller cable has a high-resistance connection, it will get hot without the engine running, but with the high-current accessories turned on. Those include the heater fan and head lights. Together those could draw over 20 amps.

I did run into one many years ago that had an intermittent failure to crank. At one point, the throttle cable became hot and melted to its casing. That cable was the only metal connection between the body and the engine, and it was the only way the 200-plus amps could try to return from the starter. The owner had bolted the heavy negative cable to the fender instead of to the engine block.

Very often you'll find a secondary ground cable from the firewall to the back of the engine. That is to reduce radiated signals that would be picked up by an AM radio. Those ground cables are not meant to handle the high starter current, but they're going to try to do that if the battery cable has a bad connection. When that happens and current can't find its way through the proper route to get back to the battery, it looks for an alternative route, and that will be through that ground strap, into the body sheet metal, and then through the smaller negative wire that is smoking. The additional symptom here is the starter would crank slower than normal, and it would for sure overheat that little wire.

Also remember all the other loads on the car have their return currents going through that small wire. The ignition system doesn't draw very much current, but when that wire already has some objectionable resistance in it, the current flow is going to cause a drop in voltage. The more things you turn on, the higher that voltage drop will be. Suppose, for example, you put a voltmeter between the negative battery cable clamp, and a paint-free point on the body, you're supposed to see 0 volts because electrically, they're the same point in the circuit. But when you have resistance in that wire, you're going to find some voltage. When you turn on the head lights, that voltage might go up to, lets say, two volts. With the engine not running, so the charging system isn't in the picture, you start out with 12.6 volts from a good, fully-charged battery. When you lose two volts in the ground wire, that leaves you with only 10.6 volts to run those head lights. Now turn on the heater fan on a higher speed. If that draws another ten amps, similar to a pair of low-beam head lamps, now you have twice as much current flow through that bad connection, so you have twice as much voltage drop. Now it's four volts, leaving you with 8.6 volts to run those systems.

Another way to verify this is to watch the brightness of the head lights change when you change speeds for the heater fan. That loss of voltage also applies to the ignition coil. Breaker-point systems are relatively forgiving compared to some electronic ignition systems, but with the voltage applied to the ignition coil lower than it should be, the resulting maximum spark voltage will also be too low.

At this point it's important to understand an ignition coil never develops more voltage than is needed to fire the spark plug. The coil may be able to develop 18,000 volts, but if it takes only 15,000 volts to cause the spark to jump the gap, once that occurs, the voltage will not build up any higher. There is always at least a few thousand volts extra available, but it's only developed if it's needed.

Now, you could have seven spark plugs that fire with 15,000 volts, and one that at times needs 15,500 volts. If the ignition coil can develop up to 18,000 volts, there will be no problem and no symptom. But now you have that voltage drop in the smaller ground wire, or in the properly-connected large ground cable going to the engine. Thinking back to the story where we had two volts drop across the ground wire, that's roughly a 15 percent drop in primary voltage to the coil. That drops the maximum voltage it can develop to 15,300 volts. It will still fire the seven spark plugs, but when that last one needs 15,500 volts, it's going to misfire. If you turn on more loads like the heater fan or brake lights, there will be more volts dropped across the negative wire, less voltage feeding the ignition coil, and pretty soon, even more spark plugs misfiring.

If you'd care to measure the voltage across that smoking wire under different conditions, I'd be interested in knowing what you find.